1. Studies on Genes that Increase the Quantitative Traits of Plants
For raising new varieties that are agriculturally useful, various breeding methods have been practiced, two examples of which are crossbreeding that involves crossing two plants and selecting the progeny and mutation breeding that induces mutation in plants. In recent years, genetically modified plants are also raised by introducing useful genes and causing their functions to be expressed. Effective for this purpose of raising new varieties is a method of accumulating genes that impart superior properties but under the circumstances where further improvements in crop productivity are desired, the availability of genes that can be used is far from being satisfactory and it is especially desirable to identify genes that govern high-yielding and other quantitative traits.
With the recent progress of techniques in molecular biology, it has become possible to perform gene analyses of quantitative traits using DNA markers. Active studies are also being made to clone agriculturally useful genes by techniques in molecular biology using genetic maps. In organisms whose genetic maps have been constructed, attempts are being made to perform techniques such as a linkage analysis for a trait that shows a particular phenotype and an associated marker and the subsequent chromosomal walking to thereby identify the physical position of the gene that governs the trait and then isolate the gene (this technique is called “map-based cloning”). However, the region including the gene that governs a particular quantitative trait can usually be specified only roughly and what can be identified is simply a DNA fragment which theoretically includes a lot of genes. It is by no means easy to identify the gene of interest on a fragment small enough to be cloned or one that is small enough to be transferred into a plant by transformation. The procedure of preparing a detailed genetic map, specifying the gene of interest based on the map information, and cloning the desired gene involves a prolonged time and much labor. Actually, there are cases in which genes capable of increasing quantitative traits were cloned by map-based cloning (Non-Patent Document 1: Ashikari et al. 2005; Non-Patent Document 2: Miura et al. 2010) but their number is quite limited.
Oryza longistaminata (O. longistaminata), a wild rice species native of Africa, is known to have the same A genome as the cultivated species Oryza sativa (O. sativa L) but show a larger biomass than the latter. The present inventors raised BC7F6 line No. 645 with increased growth in the process of introducing the long anther of O. longistaminata into the rice cultivar Shiokari. They then successfully applied map-based cloning to narrow down the increased growth imparting region to within approximately 180 kb in the farthest end portion of chromosome 7. Subsequently, the inventors determined the nucleotide sequence of approximately 82 kb of that region and investigated transformants created on the basis of the thus determined sequence but they were unable to obtain transformants showing increased growth (Non-Patent Document 3).
2. Clock-Associated Genes in Plants
As regards clock-associated genes in plants, three genes have been discovered in a study using Arabidopsis and they are CIRCADIAN CLOCK ASSOCIATED 1 (CCA1), LATE ELONGATED HYPOCOTYL (LHY), and TIMING OF CAB EXPRESSION 1 (TOC1). It has been found that a mechanism underlying the circadian clock of plants is a feedback loop for the expression of these genes, among which the TOC1 gene is known as one of pseudo-response regulators (PRRs). On the following pages, pseudo-response regulators are designated by the acronym PRR. Currently known PRR genes that have been identified in Arabidopsis are five, i.e., PRR3, PRR5, PRR7, and PRR9 in addition to TOC1 (PRR1). It was also found that PRR9, PRR7, PRR5, PRR3 and PRR1 (TOC1) are responsible for the circadian phenomenon as the result of their expression levels being elevated and attenuated in the order written (Non-Patent Document 4: Matsushika et al. 2000).
Following that discovery, five orthologs corresponding to the PRR genes of the dicotyledonous Arabidopsis were identified in the monocotyledonous rice and shown to display a circadian rhythm as does Arabidopsis. Further, these orthologs of rice, i.e., OsPRR1, OsPRR37, OsPRR59, OsPRR73, and OsPRR95, were mapped on chromosomes 1, 7, 11, 3 and 9, respectively, on the genome of rice (Non-Patent Document 5: Murakami et al. 2003). It was also reported that introduction of a construct that controls the expression of rice OsPRR37 cDNA by a promoter of the Arabidopsis PRR7 gene into a mutant of the Arabidopsis PRR7 gene led to a functional supplementation (Non-Patent Document 6: Murakami et al. 2007).
A comparison of an expression profile showed that the OsPRR gene of the Japonica rice variety Nipponbare was quite similar to that of the Indica rice Kasalath, indicating that the gene is well conserved in both Japonica and Indica varieties (Non-Patent Document 7: Murakami M et al. 2005).
Concerning PPR genes, it has been reported that by linking constitutive promoters to the said genes, the yield of plants increased. Two specific known cases are as follows: when a construct in which a promoter capable of constitutive expression in rice (GOS2 promoter) was linked to the tomato-derived structural gene PRR2 was introduced into rice, its yield increased (Patent Document 1); and when a construct in which a constitutive promoter (RICE ACTIN promoter) was linked to the Arabidopsis-derived PRR5 gene was introduced into rice, the number of rice culms increased and so did the plant height (Patent Document 2). To date, however, no case has been reported where researchers focused on PRR promoters.